1. Field of the Invention
The present invention relates to a spark plug used for an internal combustion engine.
2. Description of the Related Art
A spark plug used for an internal combustion engine, such as an automobile engine, includes a center electrode extending in a direction of an axial line thereof, an insulator disposed radially outside the center electrode, a cylindrical metal shell disposed radially outside the insulator, and a ground electrode having a base end portion joined to a leading end surface of the metal shell. The ground electrode has a substantially rectangular shape in cross section, and the inner surface of the leading end portion thereof is disposed to face the leading end portion of the center electrode. As a result, a spark discharge gap is defined between the leading end portion of the center electrode and the leading end portion of the ground electrode.
In recent years, tips (noble metal tips) containing a noble metal alloy have been attached to the leading end portion of the center electrode and the leading end portion of the ground electrode, respectively. Attaching the noble metal tips improves spark wear resistance. Particularly, a prism-shaped noble metal tip is welded to an axial-side tip surface of the ground electrode and protrudes toward the axial line of the center electrode in order to improve ignitability or spark propagation capability (see JP-A-61-45583 and JP-A-2002-324650, for example).
However, if the noble metal tip protrudes from the second end surface of the ground electrode toward the axial line as mentioned above, the protruding end portion of the noble metal tip is located apart from a base material of the ground electrode. Consequently, heat dissipation property (heat conductance from the ground electrode to the metal shell) will become unsatisfactory, and a higher temperature will be easily reached. Particularly in engines used in recent years, combustion temperature has increased, and, as a result, the leading end portion of the ground electrode is outwardly exposed to higher temperatures.
In this state, the noble metal tip and the ground electrode repeatedly thermally expand and shrink. Further, due to a dimensional difference in thermal expansion and shrinkage between these two elements, a stress difference occurs in the boundary between the noble metal tip and the ground electrode when viewed from the distal end surface of the ground electrode. As a result, the ground electrode and noble metal tip are subject to deformation or distortion. Therefore, oxygen readily invades from the boundary, and an oxide scale is liable to be formed thereon. Therefore, oxidation resistance and peel resistance may be reduced due to the oxide scale formed in the welded portion.